1. Field of the Invention
This invention relates to an exposure apparatus for the manufacture of semiconductive integrated circuits, and in particular to an exposure apparatus provided with a levelling mechanism for bringing the surface of a wafer into exact coincidence with an exposure reference plane.
2. Related Background Art
In the lithography process in the manufacture of semiconductive integrated circuits, a reduction projection type exposure apparatus of the step and repeat type, i.e., a so-called stepper, bears the main role in the lithography process. In such stepper, it is necessary to enhance the resolution limit of a projection lens correspondingly to the minimum line width of circuits formed, in the order of a submicron. The minimum line width becomes more minute year after year. At the same time a very severe requirement for satisfying both a great numerical aperture (N.A.) and a wide exposure field is increasing. However, a projection lens of a great numerical aperture (N.A.) and of a wide exposure field necessarily has a small depth of focus and therefore, if in a portion of any shot area on a wafer, inclination should occur relative to the projection image plane, it will become difficult to always effect accurate focusing on the whole surface in the exposure field. So, by the use of a horizontal position detecting optical system as disclosed, for example, U.S. Pat. No. 4,558,949, the inclination relative to the projection image plane is detected in each shot area on the wafer, and likewise, by the use of a stage device (wafer stage) as disclosed in U.S. Pat. No. 4,770,531, a plurality of predetermined points (e.g. three operating points) on a levelling stage are driven, whereby the angle of inclination of the levelling stage is controlled so that the inclination of each shot area becomes zero. When the levelling stage is to be inclined in any direction relative to the projection image plane, use is made chiefly of a two-point driving system in which one operating point is fixed and the remaining two operating points are driven.
In the above-described wafer stage, the design is such that when the stage is in a predetermined neutral state (a state in which, for example, the operating points are at the center of the movement stroke in Z-direction), the plurality of points on the levelling stage are positioned in an exposure reference plane containing the measuring axis of a laser interferometer of the wafer stage (the center axis of a laser beam) and the exposure reference plane coincides with the surface of a reference wafer placed on the levelling stage and also coincides with the projection image plane (the imaging plane) of a mask pattern formed by a projection lens.
As a result, by the use of a levelling mechanism comprising the above-mentioned horizontal position detecting optical system and the wafer stage, the surface of the shot area is brought into exact coincidence with the exposure reference plane or the imaging plane, whereby the projected image of the circuit pattern of a mask or reticle (hereinafter simply referred to as the "reticle") is projected and exposed with a high resolution without causing any partial focus deviation in the exposure field.
However, when replacement of a wafer or a wafer holder is effected in the stepper having the levelling mechanism of this type, the thicknesses of the wafer and the wafer holder, i.e., the thickness of the portion above the levelling stage, may be varied by an increase in the total thickness variation (hereinafter referred to as "TTV") of the wafer itself resulting from the tolerance of manufacture, manufacturing error, etc. Therefore, a deviation in Z-direction (focus deviation) occurs between the imaging plane and the surface of the wafer and also, a deviation in Z-direction occurs between a surface which provides the reference of the inclining movement of the levelling stage (hereinafter referred to as the "levelling reference surface") and the surface of the wafer.
Of these two deviations in Z-direction, the focus deviation can be eliminated by moving a Z-stage, but the deviation between the levelling reference surface and the surface of the wafer remains. In a state in which this deviation remains, the levelling of any shot area on the wafer is effected with the levelling stage inclined, for example, by an angle 1/4, and there has been a problem of lateral deviation of the center of the shot area relative to the coordinates system XY, in which the distance .delta. between the center of this shot area and the apparent center of pivotal movement of the levelling stage is a factor, i.e., so-called cosine error .DELTA.Ce (.DELTA.Ce =.delta..multidot. (1 - cos.theta.), and a problem of lateral deviation in which the amount of deviation .nu. in height between the center of the shot area and the center of pivotal movement of the levelling stage is as a factor, i.e., so-called sine error .DELTA.Se (.DELTA.Se=.nu./2 .multidot. sin.theta.).
Usually, the angle of inclination .theta. of the wafer is 1' at greatest and the distance .delta. is of the order of 220 mm (in the case of 8 inches) at greatest and thus, cosine error .DELTA.Ce is of the order of 0.01 .mu.m at greatest. Also, the tolerance of the thickness variation of the wafer and wafer holder is .+-.75 .mu.m and TTV is of the order of 20 .mu.m and therefore, sine error .DELTA.Se is of the order of 0.028 .mu.m at greatest. The amount of lateral deviation comprising the sum of these errors is nevertheless too great for the positioning accuracy (of the order of 0.03 .mu.m) required for the wafer stage of the stepper of this type. To correct these errors, the alignment between the projected image of the pattern and the pattern already formed on the wafer must be effected again after levelling is effected in each shot area, and this has led to the problem that the throughput is reduced.